1. Technical Field of the Invention
The present invention relates to signal transformation networks and, more particularly, relates to passive multi-port radio frequency signal transformation networks that adjust phase and amplitude balance on the lines.
2. Description of the Related Art
A number of radio communication systems, most notably mobile and portable satellite systems, require small circularly polarized antennas on the subscriber unit. These small circularly polarized antennas, such as orthogonally polarized dipole antennas, square or circular microstrip patch antennas or twisted crossed loop antennas require a 90.degree. phase shift between two excited modes of the antenna. Most of these antennas, such as the orthogonally polarized dipole antennas and the twisted crossed loop antennas, are also inherently balanced structures that require a balun (balanced-unbalanced network) for a balanced feed. The balun transforms an unbalanced transmission system from a radio transceiver to a balanced feed system for the antenna. Most external baluns and phase shifters/power dividers known in the art are prohibitively bulky and introduce excess signal loss.
Self-phasing versions of some of these circularly polarized antennas can be used to generate circularly polarized radiation without an external phase shifter. This is done by slightly shifting the resonant frequencies of the two modes away from the nominal frequency in opposite directions, so that the reactive components of the modes'input impedances cause appropriately phased currents to result from a single applied voltage source. A nearly square patch antenna probe-fed along a diagonal is an example of this sort of structure, as is the twisted crossed loop antenna with a difference in lengths between the loops. The self-phased antenna, however, is typically frequency bandwidth limited more than an externally phased antenna (non-self phased antenna). The self-phasing of the self-phased antenna limits the bandwidth more than any other features, such as, for example, impedance match features and pattern features, would limit the bandwidth of an antenna. An externally phased antenna leaves more degrees of freedom than a self-phased antenna to accomplish other antenna performance goals, such as a better antenna pattern. A better antenna pattern achieves better gain over a range of angles allowing a mobile or portable satellite transceiver to have better omnidirectional performance.
Improvements are needed for externally phasing a non-self-phased antenna. Both the function of providing a balanced-unbalanced network and providing a phase shifting network are required external to a non-self-phased antenna. A compact, high performance and low cost network is desired for providing phase shift and converting between a balanced and unbalanced line. More compact portable satellite transceivers can thereby be made available in the consumer market.